In some instances it is desirable to provide a radio frequency front-end filter. In the past ceramic filters and SAW filters have been used as front-end radio frequency filters. There are problems with SAW filters in that such filters start to have excessive insertion loss above 2.4 gigahertz (GHz). Ceramic filters are large in size and can only be fabricated with increasing difficulty as the frequency increases.
A basic FBAR device 100 is schematically shown in FIG. 1. The FBAR device 100 is formed on the horizontal plane of a substrate 110. A first layer of metal 120 is placed on the substrate 110, and then a piezoelectric layer 130 is placed onto the metal layer 120. The piezoelectric layer can be ZnO, AIN, PZT, any other piezoelectric materials. A second layer of metal 122 is placed over the piezoelectric layer 130. The first metal layer 120 serves as a first electrode 120 and the second metal layer 122 serves as a second electrode 122. The first electrode 120, the piezoelectric layer 130, and the second electrode 122 form a stack 140. A portion of the substrate 110 behind or beneath the stack 140 is removed using back side bulk silicon etching. Most commonly, the back side bulk silicon etching can be done in one of two ways—either using deep trench reactive ion etching (“DRIE”) or using a crystallographic-orientation-dependent etch (“CODE”), such as KOH, TMAH, and EDP.
The FBAR device shown in FIG. 1 is formed using DRIE. The resulting structure is a horizontally positioned piezoelectric layer 130 sandwiched between the first electrode 120 and the second electrode 122 positioned above an opening 150 in the substrate. The FBAR is a membrane device suspended over the opening 150 in a horizontal substrate. The sidewalls of the opening 150 are substantially perpendicular to the piezoelectric layer 130. There are problems associated with forming FBARs using DRIE. One of the main problems is that forming an FBAR using DRIE is not conducive to mass production. The DRIE process is a single-wafer process and the typical etch rate is 4 to 10 um/minute. The manufacturing throughput using DRIE is low since the process is conducted on only one wafer at a time and since the etch rate is low. This results in costly FBARs.
Another basic FBAR device 200 is schematically shown in FIG. 2. The FBAR device 200 is formed on the horizontal plane of a substrate 110 made from bulk (100) silicon wafers. A first layer of metal 120 is placed on the substrate 110, and then a piezoelectric layer 130 is placed onto the metal layer 120. The piezoelectric layer can be ZnO, AIN, PZT, or any other piezoelectric materials. A second layer of metal 122 is placed over the piezoelectric layer 130. The first metal layer 120 serves as a first electrode 120 and the second metal layer 122 serves as a second electrode 122. The first electrode 120, the piezoelectric layer 130, and the second electrode 122 form a stack 140. A portion of the substrate 110 behind or beneath the stack 140 is removed using back side bulk silicon etching using a CODE, such as KOH, TMAH, and EDP. Back side bulk silicon etching produces an opening 250 in the substrate 110. Etching using a CODE results in a sloped sidewalls, such as sloped sidewall 251 and sloped sidewall 252. Although using a CODE has a higher manufacturing throughput, the resulting sloped sidewalls, such as 251, 252 add to the amount of space needed to produce an FBAR device. In other words, the number of devices produced per wafer drops dramatically. For example, an FBAR that is 200 μm square (dimension L=200 μm) would have an extra 300 μm on each side (dimension S=300 μm) devoted to the sidewalls of the opening 250. The area on the substrate per FBAR device would be 640,000 square μm. The area of a wafer used with straight sidewalls would be 40,000 square μm. Thus the density of FBAR devices formed with CODE processes would be approximately 1/16th the density of FBAR devices having sloped sidewalls 251, 252. The use of CODE processes would result in a higher throughput, but would drop the number of devices that could be formed on each wafer drastically.
Thus, there is need for an FBAR device and a method for producing an FBAR device that lends itself to high manufacturing throughput and also has high number of devices per wafer. There is also a need for a lower cost FBAR device. There is still a further need for an FBAR device that can be manufactured reliably. There is also a need for a method of fabricating an FBAR device having good, reliable performance characteristics.
The description set out herein illustrates the various embodiments of the invention and such description is not intended to be construed as limiting in any manner.